Snapping in 3D indicates which point, edge or surface is going to be picked.
The available snap points are dependent on which tool (Measurement, Markup or Clip Plane) you are using.
Snapping preferences can be defined per tool (Measurement, Markup or Clip Plane). When the tool is activated a toolbar will display at the bottom of the screen.
The default setting is Auto, which will snap to the closest edge, point or face that your cursor is near.
You are not limited to only selecting 1 snap preference. When 2 preferences are selected, the application will snap to the closest preference (edge, point or face) that your cursor is near.